Horseweed (Conyza canadensis) is a dicot weed in the Asteraceae family. In Ontario this weed first evolved resistance to Group D/22 herbicides in 1993 and infests Peaches. Group D/22 herbicides are known as PSI Electron Diverter (Photosystem-I-electron diversion). Research has shown that these particular biotypes are resistant to paraquat and they may be cross-resistant to other Group D/22 herbicides.

The 'Group' letters/numbers that you see throughout this web site refer to the classification of herbicides by their site of action. To see a full list of herbicides and HRAC herbicide classifications click here.

Rosettes of Conyza canadensis can be easily controlled by products containing glyphosate. Infestations of paraquat resistant Conyza canadensis are decreasing in Ontario because orchard growers have switched to glyphosate.

Greenhouse, and Laboratory trials comparing a known susceptible Horseweed biotype with this Horseweed biotype have been used to confirm resistance. For further information on the tests conducted please contact the local weed scientists that provided this information.

Genetics

Genetic studies on Group D/22 resistant Horseweed indicate that the inheritance is determined by a one gene, dominant, nuclear trait. There may be a note below or an article discussing the genetics of this biotype in the Fact Sheets and Other Literature

Mechanism of Resistance

The mechanism of resistance for this biotype is either unknown or has not been entered in the database. If you know anything about the mechanism of resistance for this biotype then please update the database.

Relative Fitness

There is no record of differences in fitness or competitiveness of these resistant biotypes when compared to that of normal susceptible biotypes. If you have any information pertaining to the fitness of Group D/22 resistant Horseweed from Ontario please update the database.

The Herbicide Resistance Action Committee, The Weed Science Society of America, and weed scientists in Ontario have been instrumental in providing you this information. Particular thanks is given to Susan Weaver for providing detailed information.

In the decade since the introduction of herbicide tolerant (HT) crops, an increasing number of weed species have been identified with resistance to the widely utilized herbicide, glyphosate. In the USA, there are now 14 weed species with confirmed resistance to the non-selective herbicides utilized in these systems. Similarly in Canada, there are 5 weed species that exhibit glyphosate resistance (GR); two of which were documented in the last year alone. Conyza canadensis has developed GR in both countries and its capacity for long-distance dispersal raises the potential of cross-border movement of herbicide resistant traits. The objective of this research was to examine the origins of GR C. canadensis populations from SW Ontario and explore their mechanism(s) of resistance. New and historic populations of C. canadensis collected throughout the counties comprising the most SW portion of the province of Ontario, as well as from bordering states of the USA. Eight microsatellite markers were used to characterize the relatedness of these populations and the target site gene for glyphosate (EPSPS2) was also sequenced. Dose-response studies were carried out to ascertain or confirm the resistance levels of the various populations. Results from the microsatellite and dose response studies indicate that the majority of Canadian GR populations are distinct from the American populations tested and exhibit notably higher resistance. Sequencing of EPSPS2 confirmed that, while GR in American populations is mediated by non-target site mechanisms, the predominate mechanism of resistance in Canadian populations of C. canadensis is target site mediated. These results represent the first report of target site mediate resistance to glyphosate in C. canadensis and clearly expose geographically distinct mechanisms of resistance to a given active ingredient on a regional scale within a single highly mobile species..

Conyza canadensis (CC) and Conyza bonariensis (CB) are troublesome weeds around the world. Extensive use of herbicides has led to the evolution of numerous Conyza spp. herbicide-resistant populations. Seeds of 91 CC and CB populations were collected across Israel. They were mostly found (86 %) in roadsides and urban habitats, two disturbed habitats that had been dramatically impacted by human activities, thus we classify these species as anthropogenic. Although pyrithiobac-sodium was only used in cotton fields, 90 % of Conyza spp. populations were identified as pyrithiobac-sodium resistant, suggesting possible natural resistance to pyrithiobac-sodium. CC21 and CC17 C. canadensis populations were highly resistant to all tested ALS inhibitors due to a substitution in the ALS gene from Trp574 to Leu. They were also atrazine resistant due to a substitution in the psbA gene from Ser264 to Gly. The high level of imazapyr and pyrithiobac-sodium resistance observed in the CC10 population was due to an Ala205 to Val substitution. However, high resistance to sulfometuron methyl and pyrithiobac-sodium in population CC6 was due to a point mutation at Pro197 to Ser. All resistant plants of CC21 population showed both psbA (Ser264 to Gly) and ALS (Trp574 to Leu) substitutions, leading us to the conclusion that the attempt to overcome resistance to one mode of action by overuse of another will most likely lead to multiple herbicide resistance. Furthermore, we concluded that only individuals that carry both mutations could survive the shift between the two modes of action and overcome the fitness cost associated with the PSII resistance..

Glyphosate resistant (GR) Canada fleabane (horseweed) has quickly spread across southwestern Ontario and is a difficult weed to control in GR crops. Glyphosate dimethylamine (DMA)/2,4-D choline (Enlist Duo®TM Dow AgroSciences LLC), a new herbicide premix developed by Dow Agro Sciences, provides control of GR and other problematic weeds. The objective of this study was to compare single and sequential applications of glyphosate DMA/2,4-D choline for the control of GR Canada fleabane in GR corn. Three single applications of glyphosate (DMA)/2,4-D choline (1720 g.ae.ha-1) were evaluated: (1) preplant (PP) applied to Canada fleabane up to 10 cm diameter/height, (2) postplant 1 (POST 1) applied when Canada fleabane was up to 20 cm tall and (3) postplant 2 (POST 2) applied up to 30 cm tall Canada fleabane. Four sequential applications were also examined:(1) PP followed by (fb) POST 1, (2) PP fb POST 2, (3) POST 1 fb POST 2 and (4) PP fb POST 1 fb POST 2. The single applications provided 69%-86% control of the GR Canada fleabane while the sequential applications increased control to 92%-100%. Three applications did not provide an increase in control over a sequential two-pass application at 8 weeks after the application (WAA). Results from this research indicate that a sequential 2-pass application of glyphosate DMA/2,4-D choline provided acceptable control of GR Canada fleabane in corn..

(Conyza bonariensisandConyza canadensis) recorded in Rio Grande do Sul State, however resistance ofConyza sumatrensisto glyphosate is registered only in Paraná State. This report has the objective of stating the presence ofConyza sumatrensisresistant to glyphosate biotypes in Rio Grande do Sul State Brazil. To support this report, researches to evaluate the occurrence of glyphosate resistance in horseweed biotypes, in Rio Grande do Sul State, and for that, seed samples were collected in soybean areas with unsatisfactory control, 25 biotypes. These biotypes were preliminary evaluated for resistance to glyphosate, applying a dose of 720 g/ha. Seeds from this first test of two selected biotypes of horseweed, which were not controlled in the first trial, with contrasting degree of susceptibility, were chosen to develop the second trial. The selected biotypes were grown after herbicide application (first trial) to seed production. It was also made, from these biotypes, herbarium specimens, which were sent to the Department of Biology of the Federal University of Santa Maria, where it was cataloged and classified by Dr. Thais Scotti Canto Dorow, plant systematic specialist, asConyza sumatrensis(Retz.) E. Walker (TELES; BORGES; HEIDEN, 2013) and subsequently deposited in the herbarium, SMDM under numbers SMDM 13950, 13951, 13952 and 13953. The biotypes selected, derived from seeds produced were evaluated by dose-response curves, using eight herbicide doses (0, 45, 90, 180, 360, 720, 1,440 and 2,880 g/ha) applied at growth stage of 3 to 4 leaves and/or 0.5 to 1.0 inch height. The experiments were conducted in a greenhouse in a completely randomized arrangement of treatments with four replications. The results indicate that the susceptible biotype to glyphosate has reduced 50 % of dry matter when used dose of 34.3 g/ha, compared with the resistant biotype, that requires 143.6 g/ha for the same control. The dose that provides 50% control of the susceptible population was 172 g/ha, compared with the resistant biotype that needs 2,028 g/ha for the same control, showing a resistance factor of 11.8..

Changes in leaf anatomy were examined in two S. African populations of Conyza canadensis of which one was presumed to be resistant (CCPR) and the other susceptible (CCS) to glyphosate. Glyphosate was applied to plants, which were grown from seed collected from these populations, at rates of 1, 2 and 4 kg a.e. ha-1 of TOUCHDOWN® [active substance: glyphosate trimesium salt, 500 g L-1] that are equivalent to 2, 4 and 8 L ha-1 of the herbicide Touchdown. Leaf samples for the light microscope (LM) analysis were collected 3, 7 and 24 h after treatment (HAT). Changes in chlorophyll and shikimate content of leaf material were also examined. Changes in the palisade and pith tissue of leaves were not detected in the investigated populations at 3 and 7 HAT. However, at 24 HAT the different herbicide doses caused changes in leaf anatomy. These changes (injuries) were detected in the CCS at all tested doses, but in the CCPR population of C. canadensis the injuries were observed at only the two highest rates, 2 and 4 kg a.e. ha-1. Chlorophyll and shikimate contents indicated significant differences between the treated and untreated plants of susceptible population only. Difference in glyphosate resistance between the CCS and CCPR populations was confirmed with an index of resistance of 1.58. This value of the index of resistance indicates that CCS population is 1.58 times more susceptible to glyphosate compared to CCPR population..